Resumen de Antibiotic amendment for suppression of indigenous microflora in feed sources for an Escherichia coli auxotroph lysine assay

I. B. Zabala-Díaz, S. C. Ricke, A. M. Erickson

• Growth responses of lysine auxotrophic mutants of Escherichia coli have been used as a measurement of bioavailable lysine in protein sources and animal feeds. Sterilizing feed samples by autoclaving to eliminate non‐specific background growth of indigenous feed micro‐organisms prior to conducting the bacterial assay may introduce chemical and physical alterations to the feeds, influencing the estimation of available feed lysine. In this study, an antibiotic‐ and antifungal‐supplemented medium was constructed to support growth of an E. coli lysine auxotroph assay organism, and was tested for its ability to repress indigenous bacterial and fungal growth in feed samples. To determine which antibiotics to include, an ampicillin‐sensitive E. coli lysine mutant strain (ATCC no. 23812) was screened for antibiotic resistance and transformed with a plasmid carrying an ampicillin resistance gene. Maximum optical density quantitative response of the E. coli auxotroph to lysine was not altered by the antibiotic medium amendments (ampicillin, novobiocin and cycloheximide). Indigenous microfloral growth in a variety of typical animal feeds was suppressed in the presence of the antistatic agents. The estimated lysine recovery was 91·6% and 98·1% when the medium was used in an assay of available lysine in a lysine‐supplemented feed. This indicates that the antibiotic‐amended basal medium can be used for the E. coli‐determined lysine availability of a variety of animal feeds without prior sterilization of the feed sources.